There are numerous circumstances in the medical arts where it is highly valuable to determine the comparative health of an individual at a point in time, compared to others (or compared to known categories), or compared to the individual's history. In particular, this health determination often seeks to measure the individual's cardiopulmonary function, as blood circulation and gas exchange are front-line life-enabling in comparison to dietary metabolism or cognitive functions. In some aspects and as used herein, “health” refers to the cardiopulmonary vitality for humans although it will be understood that other types of health vitalities are possible.
Determining the comparative health of a human is useful in a variety of different ways. For example, these determinations can be used to determine the efficacy/side effects of a drug in a drug trial (e.g., whether the drug usage is improving the subject's health, sustaining the subject's health in view of expected degradation, or is having negative side effects on the subject's health). Comparative health determinations can also be used to obtain the efficacy of a device/procedure intervention in a clinical trial, or determine a capacity for treatment (e.g., in oncology treatments/chemotherapy, determine whether the patient sufficiently recovered from last treatment to endure more, whether the patient is being overtreated, or whether the treatment is otherwise harming the patient). These determinations can further be used in health care surveillance (e.g., tracking the day-over-day trajectory of a patient with chronic disease (e.g., heart failure or COPD), or post-surgical discharge). These determinations can additionally be used to identify individuals with undiagnosed disease in the population, or to enable early intervention (e.g., NYHA Class I Heart Failure, which is often underdiagnosed and is asymptomatic). Still further, these determinations can be used to evaluate the effectiveness of rehabilitative regimen (e.g., whether the regimen is helping the patient or whether the patient performing the regimen). Other examples are possible.
Indicators of health status are typically measured under “effort challenge” activities (activities that individuals may be reluctant to perform), such as exercise tests. In general, exercise tolerance is determined by three factors: pulmonary gas exchange; cardiovascular performance, including the peripheral vascular tree; and skeletal muscle metabolism. Methods for directly and objectively measuring an individual's cardiopulmonary functional capacity are generally invasive or arduous to set up and perform. Because of expense or inconvenience, they are not easily deployed for long-term surveillance or repeated measurement, whether in a clinical trial or for extended monitoring of a patient.
For example, objective measurements of cardiopulmonary function include ventilatory anaerobic threshold (VAT) tests. VAT measurements involve successive blood samples are drawn from a subject during exercise challenge to determine an inflection point at which metabolism moves from aerobic to anaerobic. This method involves challenges of sequential blood gas testing and setup to acquire blood samples in an exercise equipment setting.
Another objective measurement of cardiopulmonary function includes the cardiopulmonary exercise test (CPET), where the subject is fitted with a breathing mask for measuring oxygen uptake and carbon dioxide production. The mask and connected equipment are able to measure O2 (and/or CO2) content from the breath, so that an objective estimate of oxygen uptake can be made, in particular as the subject reaches a plateau of maximum exercise capacity. This method requires specialty equipment as well as a laboratory-type exercise equipment setting.
Still another objective measurement of cardiopulmonary function includes the pulmonary function test (PFT), which uses either a spirometer or a plethysmograph chamber, to measure volumes and rates of air during breathing. This method only measures breathing, and so does not capture the “cardio” side of the health assessment. Nonetheless, it presents a way for objectively measuring structural gas exchange volume and flow.
Yet another objective measurement of cardiopulmonary function includes the cardiac echo doppler test, which is a measure of the cardiac output, physical distension and valve health of a person. These measurements are not expected to change much over short periods (weeks) of time. This measurement does not address the pulmonary side of performance or skeletal muscle metabolic health.
As a substitute for these objective measurements, more easily deployed tests like the “6-minute walk test” (6MWT) are used in health monitoring and clinical studies. In the 6MWT, a clinician marks out a stretch of hallway in which the tested individual walks back and forth as far as possible in 6 minutes. The distance walked is a proxy for cardiopulmonary exercise capacity. One problem with the 6MWT is its crudeness: Settings may vary in terms of obstacles or distractions; a tested individual may walk less distance owing to something other than cardiopulmonary fitness, such as arthritis, a headache, balance issues or joint pain. Hence results are noisy and only coarsely correlated to true health. Moreover, it is inconvenient for a tested individual to travel to the clinician's location to participate in the test.
Estimators of VO2Max have been derived based on physical attribute values such as age or gender. However, these are also only coarsely accurate, and of course lend no information on day-over-day changes in the performance of an individual. For example, such estimators are useless for monitoring the status of a patient with chronic disease who may evolve a disease exacerbation over days or weeks. More recently, VO2Max estimators have begun to be developed using features from fitness trackers, mainly heart rate, in combination with physical attribute values, but these have lacked accuracy, and often still require the subject to perform a specific exercise routine during which the assessment is made.